Conventional color television receivers and display monitors usually include an average beam current limiter for the protection of the picture tube and the high voltage circuitry. When the average beam current exceeds a predetermined maximum amplitude of, for example, 1.4 milliamperes (ma), the contrast and/or brightness of the reproduced image is decreased by the beam limiter to avoid a further increase of the beam current.
FIG. 1 shows the circuit diagram of a television receiver including a commonly employed average beam current limiter. In a video signal processing channel 10, low level red (r), green (g), and blue (b) color video signal provided by a video signal processing circuit 20 are amplified by respective picture tube (or kinescope) drive amplifiers, generally indicated by reference number 30, to produce high level red (R), green (G), and blue (B) color video signal suitable for directly driving respective cathodes of a picture tube (or kinescope) 40. Although there are three drive amplifiers, one for each color, only drive amplifier 30R for producing the high level red (R) color video signal is shown for simplicity. As is indicated in FIG. 1, low level video signal processing circuit 20 may comprise an integrated circuit (IC), such as the TDA 3562 or TDA 3506 IC, available from Philips of The Netherlands. A high supply voltage for picture tube 40 is developed at the high potential end of a high voltage winding 51 of a horizontal flyback transformer 50 in response to horizontal deflection signals generated by a horizontal deflection circuit 60.
Beam limiting arrangement 70 of the television receiver shown in FIG. 1 is of the conventional type which responds to the average beam current drawn by picture tube 40. Specifically, the beam current of a picture tube 40 is sensed (or "sampled") at the junction of a resistor R1 and a capacitor C1 coupled to the low potential end of high voltage winding 51 of horizontal flyback transformer 50. Resistor R1 and a relatively high voltage supply (e.g., +200 volts) form a constant current source from which current flowing to picture tube 40 through high voltage winding 51 of flyback transformer 50 is drawn. Capacitor C1 serves to remove horizontal rate signal components. The sense point is coupled to the contrast control input of the video signal processing circuit 20 through a low pass filter circuit including a resistor R2 and a capacitor C2 and a switching diode D2. A manual contrast control circuit 80, illustratively shown as including resistors R3, R4 and R5, a potentiometer R6 and a filter capacitor C3, is also coupled to the contrast control input of the video processing circuit 20. A diode D1 clamps the junction of capacitor C2 and diode D2 to a relatively low voltage (e.g., +12 volts) and therefore protects capacitor C2 and diode D2 from the relatively high supply voltage (e.g., +200 volts) which might otherwise be applied to them at relatively low beam currents.
In operation, diode D1 is conductive and diode D2 is non-conductive at low beam currents. The voltage developed across capacitor C1 decreases toward zero as the average beam current increases. Protection diode D1 is rendered non-conductive and thereafter switching diode D2 is rendered conductive as the beam current increases. After switching diode D2 is rendered conductive, the contrast control voltage is decreased as a function of beam current and, in turn, the beam current is also decreased. Diode D2 is rendered conductive when its cathode voltage falls approximately 0.7 volts below its anode voltage. The anode voltage is equal to the contrast control voltage. The point at which diode D2 is rendered conductive and average beam current limiter 70 is activated to reduce the contrast is referred to as the "threshold" or "delay" of average beam current limiter 70. The "threshold" or "delay" of average beam current limiter 70 shown in FIG. 1 is determined by the value of resistor R1 and is fixed.
The rather low dynamic range of a picture tube may cause an overload problem when only the average beam current is limited as in the television receiver shown in FIG. 1. This is illustrated by the waveform shown in FIG. 1--1.
Waveform 1a represents a video signal (V.sub.1) for producing a large white window. Such a window results in a high amount of average beam current which puts the beam limiter into operation. Waveforms 1b and 1c show video signals producing the same amount of average beam current as the signal of waveform 1a because their envelopes cover same area. Assuming the identical contrast setting as for waveforms 1a, 1b and 1c, the staircase signal of waveform 1b produces an ultra bright picture portion, while waveform 1c produces bars of excessive brightness and hot spots on the picture tube mask. Such hot spots may cause the mask to deform in which is sometimes referred to as mask "doming". Thus, the contrast setting for waveform 1c should be lower than for waveform 1a. Unfortunately, beam current limiting arrangement 70 shown in FIG. 1 cannot differentiate between waveforms 1a, 1b and 1c shown in FIG. 1--1. This is so because the beam current is averaged by the aquadag capacitor of picture tube 40 (i.e., the capacitor formed between the inner and outer coatings of picture tube 40) and the voltage developed across capacitor C1 is related to the current which recharges the aquadag capacitor through high voltage winding 51 during horizontal retrace. The actual beam current is the sum of currents flowing through the electron guns of picture tube 40.